1. Field of the Invention
The present invention relates to a method for manufacturing a tyre for vehicle wheels, comprising the following steps: forming a casing sleeve provided with a pair of annular bead reinforcing structures spaced apart from each other axially and at least one casing ply having each of its opposite end flaps turned around one of the said annular bead reinforcing structures; transferring the casing sleeve to a shaping drum; shaping the said casing sleeve into a toroidal configuration; and applying a pair of sidewalls on to lateral surfaces of the casing sleeve shaped in this way, each of these surfaces extending radially outwards from one of the annular bead reinforcing structures. According to the aforesaid method, the manufacture of a tyre for vehicle wheels also comprises the steps of forming an outer sleeve comprising a belt structure and a tread applied in a radially external position on to the belt structure, and transferring the said outer sleeve into a centred position around the casing sleeve positioned on the shaping drum, to cause the outer sleeve to be applied to the casing sleeve as a result of this shaping. The operations associated with building the outer sleeve can be carried out in any known way and are not of any special significance for the purposes of the present invention, and are therefore not described in detail below.
The invention also relates to a building drum, and more precisely a shaping drum, for manufacturing a tyre for vehicle wheels, provided with two coaxial half-drums, integral with each other as regards rotation, which make up the drum, at least one of these half-drums being axially movable in both directions with respect to the other. Each of the said half-drums comprises a suitable expandable portion which causes one sidewall to be turned so that it is applied to the corresponding lateral surface of the casing structure; the said drum is also provided with radially expandable gripping devices for retaining the beads of the casing sleeve on the shaping drum, and with centring devices for the centred axial positioning of the casing sleeve on the equatorial plane of the shaping drum.
2. Description of the Related Art
A tyre building method widely used in the art is that commonly known as the “two-step process”. More particularly, in the tyre making process one or more casing plies are first deposited on a first cylindrical drum, usually called the “building drum” or “first step drum”, to form a cylindrical sleeve. The annular reinforcing structures at the beads are fitted on to the opposite end flaps of the casing ply or plies, which are then turned around the annular structures in such a way as to enclose them in a kind of loop.
The casing sleeve built in this way is then transferred to a second drum, called the “shaping drum” or “second step drum”, on which strips of elastomeric material for forming the sidewalls of the tyre may already have been positioned.
The casing is then shaped into a toroidal configuration to join it to an outer crown sleeve which has previously been placed in a centred position with respect to the casing sleeve, and which comprises a belt structure and a tread radially superimposed on the belt structure.
After the casing structure has been shaped, suitable expandable portions of the shaping drum, consisting for example of lever mechanisms or inflatable chambers, cause the said strips of elastomeric material to be turned so that they are applied to the lateral surfaces of the casing structure.
Methods and apparatus operating according to these principles are described, for example, in FR-A-2,093,180 and U.S. Pat. No. 3,990,931.
The critical aspect of this method consists in the fact that all the operations carried out in the second step are executed on a casing sleeve which may not have been perfectly centred in its fitting on the shaping drum; in other words, its equatorial plane may not coincide with the equatorial plane of the said drum, so that it is secured on the latter only by the securing force exerted by suitable radially expandable gripping devices on the corresponding reinforcing portions of the casing sleeve, which will be referred to simply as “beads”, as is the usual practice, in the following text.
This retaining force may not be sufficient to prevent small displacements of the beads with respect to the corresponding gripping devices, or the unwinding of the casing ply from around the annular bead reinforcing structures, particularly during the step of toroidal shaping of the beads, so that, as a result of the incorrect fitting and/or subsequent displacement of the sleeve on the drum or unwinding of the ply at the beads, a deformed shaping of the casing is possible, and consequently the completion of a tyre which has uneven geometry and must therefore be discarded, since it will be unbalanced in use.
The document EP-997,263, in the context of a two-step process, illustrates a method and an apparatus for building tyres, according to which the axial centring of the casing sleeve is ensured during its fixing to the shaping drum at the end of the step of transferring the sleeve to the drum.
However, the problem has not been fully resolved, and has become even more critical with the development of a new kind of tyre, fully described for example in U.S. Pat. No. 5,634,993. This tyre is characterized in that it has its beads, preferably with different fitting diameters and different dimensions from each other, joined to a rim having the bead support surfaces, in other words the bead bases, facing axially outwards, in other words inclined in the form of a conical surface with its vertex on the axis of rotation of the rim in a position axially external to the tyre.
This particular bead geometry increases the risks of an incorrect centring of the casing sleeve on the shaping drum.
The problem is further complicated by the fact that this tyre requires the presence of a reinforcing element on the axially inner surface of the bead, to protect the latter from friction against the metal surface of the mounting rim.
This reinforcing element is preferably assembled on to the tyre in the second building step, but the particular position in which it has to be fixed prevents the use of centring devices acting against the axially inner surface of the bead, such as those described in the document EP-997,263 cited above.